A pipe coupling device 100, as shown in FIGS. 15A, 15B, 16A and 16B, for example, as a related art, a retaining ring 104 is provided with a plate-like flange part 141 extending in a circumferential direction. A flange part 141 is moveable in a flexibility space S formed between an arm part 105a of a pressing plate 105 and a press ring 107.